Polymer blends comprising a second polymer dispersed in a matrix of a first polymer are very useful and, depending on the properties and the relative amounts of the first and second polymers, a wide variety of such polymer blends can be produced. Of particular interest are polymer blends, also referred to as thermoplastic elastomers (TPE).
One method of making the aforementioned polymer blends is by melt mixing the two different polymers after they have been polymerized to achieve a target set of properties. Examples of such processes employing vanadium catalysts in series reactor operation to produce different types of EPDM compositions are disclosed in U.S. Pat. Nos. 3,629,212; 4,016,342; and 4,306,041.
U.S. Pat. No. 6,245,856 discloses a thermoplastic olefin composition comprising polypropylene, an ethylene-alpha olefin elastomer and a compatabilizer comprising an ethylene-propylene copolymer having a propylene content of greater than 80 weight percent. In the absence of the compatabilizer, the elastomer phase is said to be uneven with particles greater than 5 microns in size, whereas the addition of the compatabilizer is said to improve dispersion such that the elastomer phase has a particle size of about 1 micron. The elastomer phase of this polymer blend is not cross-linked.
U.S. Pat. No. 6,207,756 describes a process for producing a blend of a continuous phase of a semi-crystalline plastic, such as polypropylene, and a discontinuous phase of an amorphous elastomer, such as a terpolymer of ethylene, a C3-C20 alpha olefin and a non-conjugated diene. U.S. Pat. No. 6,319,998 also discloses using series solution polymerizations to produce blends of ethylene copolymers. U.S. Pat. No. 6,770,714 discloses the use of parallel polymerizations to produce different polymeric components that are then blended through extrusion or using other conventional mixing equipment.
One problem involved in producing thermoplastic elastomeric blends by direct polymerization, particularly blends having a high molecular weight and high melting temperature crystalline phase and a high molecular weight elastomeric phase, is the disparate conditions favoring the production of the individual blend components. Thus, for example, isotactic polypropylene is widely produced commercially by a slurry polymerization process, whereas ethylene propylene copolymer with desired properties for TPE applications is commercially produced in a solution process. However, the solution process is not currently capable of producing polypropylene with high molecular weight and high melting point. Ethylene propylene copolymer elastomers have been difficult to produce using slurry-based polymerization systems since, even at low reactor temperatures, these tend to result in reactor fouling and the formation of rubbery clumps that attach themselves to the reactor agitator, thereby necessitating reactor shut-down. It is therefore difficult, if not impossible, to produce TPE material in reactor using either a slurry process (good for polypropylene) or a solution process (good for rubber).
In addition to the issue of reaction conditions, choice of polymerization catalyst is another factor that currently limits in-reactor blending of TPE materials. There is a general perception in the industry that Ziegler-Natta catalysts and metallocene catalysts are incompatible and hence there is currently no in-reactor process for producing blends of Ziegler-Natta-produced polypropylene with metallocene-produced elastomer.
One particularly useful form of thermoplastic elastomer is a thermoplastic vulcanizate (“TPV”). TPVs are normally produced by a process of “dynamic vulcanization”. Examples of dynamic vulcanization are described in the U.S. Pat. Nos. 4,130,535 and 4,311,628.
An improved process for producing TPVs is disclosed in U.S. Pat. No. 6,388,016, incorporated herein in its entirety, in which a polymer blend is produced by solution polymerization in series reactors employing metallocene catalysts and the resultant blend is subjected to dynamic vulcanization. It will, however, be seen that this improved process still relies on dynamic vulcanization to cure the elastomeric component. Moreover, there is a limit to the molecular weight of the polymers that can be produced, due to the resulting increase in solution viscosity, phase-separation of produced polymer and reactor fouling at low reaction temperature inherent in the use of solution polymerization to produce the elastomeric second polymer.
There is therefore a need for an improved, in-reactor process for producing polymer blends and, in particular, polymer blends comprising a thermoplastic continuous phase and a cross-linked elastomeric phase dispersed in the continuous phase.
U.S. Pat. No. 6,492,473 describes a mixed transition metal olefin polymerization catalyst system suitable for the polymerization of olefin monomers comprising at least one late transition metal catalyst system and at least one different catalyst system selected from the group consisting of late transition metal catalyst systems, transition metal metallocene catalyst systems or Ziegler-Natta catalyst systems. Preferred embodiments include at least one late transition metal catalyst system comprising a Group 9, 10, or 11 metal complex stabilized by a bidentate ligand structure and at least one transition metal metallocene catalyst system comprising a Group 4 metal complex stabilized by at least one ancillary cyclopentadienyl ligand. The polymerization process for olefin monomers comprises contacting one or more olefins with these catalyst systems under polymerization conditions.